1. Field of the Invention
The present invention relates to the field of recording and replaying digital data, and in particular, to a digital computer interface for simulating and transferring Compact Disc-Interactive (CD-I) data.
2. Art Background
In the music and recording industry, and increasingly in the data processing industry, it is quite common to store audio, video and text data on a compact disc and replay the same on a compact disc decoder. The compact disc is a record carrier for storing audio, video and text data as digital information according to certain formats such as the Compact Disc Digital Audio System (CDDA). CDDA provides for sampling audio data at 44,100 Hz, interleaving subcodes and others. See, System Description of the Compact Disc Digital Audio (Sony and N. V. Philips, April 1987). Subcode is an auxiliary data stream which is merged with the regular data and which has numerous functions among which are to assist in locating the beginning of the different data pieces on a compact disc and in providing a catalog of their location on the compact disc and their duration. A further vital function provided by the subcode is to convey the status of pre-emphasis in the recording so that de-emphasis can be automatically selected in the device accepting the compact disc. See, Watkinson, J. D., The Art of Digital Audio (Focal Press: 1989) pages 470-478. The compact disc decoder is a device that accepts compact discs, reads the information on discs, and converts the information thereon to: (a) analog audio signals, (b) digital audio signals and (c) subcode signals. The digital audio signals from a compact disc decoder is generated via the AES/EBU interface and its derivatives. The AES/EBU standard is a widely accepted serial transmission format for linearly representing digital audio data. See AES Recommended Practice for Digital Audio Engineering--Serial Transmission Format for Linearly Representing Digital Audio Data, Audio Engineering Society, J. Audio Eng. Soc. 33, 975-984 (1985); Digital Audio Interface CP-340--Standards of Electronic Industries Association of Japan, Engineering Department of Electronic Industries Association of Japan (September, 1987), pp. 2-6. The subcode signals are also encoded in the AES/EBU stream, and typically appear as displays on the front panel of the compact disc decoder. The compact disc decoder accepts simple commands, such as play, stop, pause, go to next track and etc. However, the compact disc decoder does not interpret information other than compact disc audio and subcodes.
More recently, the compact disc also finds application in interactive use, referred to as Compact Disc-Interactive (CD-I). A CD-I decoder is a device having a video display screen, an audio output, an input device (mouse, keyboard, or pointer), and an interface to various storage media including at least a compact disc decoder and optionally a floppy or hard disc. See D. C. Geest Standardization of CD-I Technical Paper presented at Microsoft Conference, Seattle, March 1986. Also see, for example, U.S. Pat. No. 4,794,465. In contrast to the compact disc decoder, the CD-I decoder interprets the program and data on the compact disc as well as compact disc audio and subcodes. The CD-I decoder typically receives data from the compact disc player over an AES/EBU interface and sends thereto simple commands, such as stop, play, jump, go to next track and etc.
The process of manufacturing or pressing a compact disc is expensive, and the turnaround time ranges from a few days to several weeks, depending on the availability of the factory. The digital information on a compact disc is essentially programs and data. If there are errors in the programs or in the logic of the programs or in the data itself, it would be useful to detect and correct such errors before the mass production of the compact disc begins.
It is therefore an object of the present invention to simulate with a digital computer the signals to a CD-I decoder accurately. It is a further object of the present invention to provide an interface for a digital computer for storing and replaying encoded CD-I data directly to and from commercial devices that support an AES/EBU standard or a consumer version thereof.
As will be described more fully below, the present invention provides a hardware solution to the task of simulating CD-I data. The present invention permits a digital computer to store digital data from commercially available CD-I decoder and to replay the same in real time. To store and replay real time, high quality digital audio, digital audio must be sampled at 44,100 Hz or approximately once every 22 microseconds. However, the input to and the output from a digital computer is bursty, and delay between data outputs and data inputs ranges from a few milliseconds to a full second. This delay arises because of contention for the use of buses and memory by different parts of a digital computer and disk seek delays and rotational latencies. It follows that the normal data inputs to and outputs from a digital computer is unacceptable for real time, high quality audio input and output. As such, a timebase correction is necessary to compensate for the bursty input and output of a digital computer. Timebase correction refers to the technique of running the computer or machine in advance of real time such that the correctly timed output signals will enable the digital audio to be sampled at every 22 microseconds or at 44,100 Hz. See Watkinson J., The Art of Digital Audio, (Focal Press: 1988), pages 76-84.
It is yet another object of the present invention to incorporate timebase correction in an AES/EBU audio interface to a digital computer for storing and replaying digital audio without further software assistance.
It is yet another object of the present invention to convert parallel, bursty computer data into synchronized data format suitable for transfer between computers coupled to a system bus.
There are several commercially available compact disc simulation systems on the market. Among them are TOPiX CD ROM Publishing System by Optical Media International, CD Publisher Trademark by Meridian Data, Disc Architect by TMS and The CD Simulator by Electroson. It is believed that none of the prior art compact disc simulation system provide real time digital audio. Furthermore, most of the prior art system rely on software to interleave the subcodes.